Previous backlight illumination sources for flat panels include cold cathode fluorescent lamp (CCFL), hot cathode fluorescent lamp (HCFL), electro luminescent (EL), organic light emitting diode (OLED) which is generally a broad area emitter, and tungsten and tungsten halogen filament lamps and arc lamps in conjunction with woven fiber optics.
In comparison to these other backlight technologies, LEDs combine the advantages of low voltage operation, high reliability, long life, wide color gamut, high contrast, shock and vibration safe, contain no mercury, have high luminous efficiency, operate over a wide ambient temperature range between −40 and +85° C., rapid switching speed, dimmable by pulse width modulation (PWM), reduced blur (owing to short response time relative to LCD switching time), and are particularly well suited for edge lighted illumination. While these advantages make the use of LEDs very attractive for LCD backlighting applications, a number of challenges still remain. These challenges include luminous efficiency, cost, system design complexity, including the need for new power supplies and controllers, independent closed loop intensity control of red, green, blue (RGB) based systems, and thermal control requirements associated with the use of RGB based systems.
The prior art shows an approach replacing CCFLs by the use of red, green, and blue LEDs in combination. While this approach has the benefits of improved color gamut and contrast relative to CCFLs, it suffers from thermal instability, primarily due to the strong red, and to a lesser but still significant extent, the green dependence on the LED intensity on temperature.
The prior art uses a number of photodiodes distributed throughout the lighting system to address this problem, but the photodiodes are separate from the LED source to control the LED intensity. However, this approach adds considerable cost and complexity to the overall system.
This temperature dependence is addressed by the present invention by use of closed loop feedback control, which maintains a constant brightness both as a function of time and temperature. The present invention can work with either LED excited phosphors or with standard red, green and blue LED die.
In view of the many shortcomings of the prior art, it is a principal object of the present invention to provide an improved backlighting source employing LEDs for use in illuminating LCD flat panel screens and the like.
It is another object of the present invention to provide an LED based source of illumination having improved color gamut and contrast.
It is yet another object of the present invention to provide an LED based linear light source.
It is still another object of the present invention to provide an LED based illumination source having relaxed thermal management needs and simplified control requirements.
It is yet another object of the present invention to provide an improved LED board architecture by which closed loop intensity control is implemented and high thermal performance achieved.
Another object of the present invention to use multiple emitters on a single LED board to significantly reduce cost over the approach of using prepackaged devices.
Yet another object of the present invention is to provide an optical system in a narrow and short package which allows for low divergence output in a direction normal to an LCD panel.
Other objects of the present invention will be obvious and will appear hereinafter when the following detailed description is read in connection with the accompanying drawings.